1. Field of the Invention
The present invention relates to a method for the manufacture of active matrices for electro-optical display devices. It can be applied especially to the manufacture of high-density flat screens. It can be applied more generally to matrix display devices used in direct viewing or in projection.
To put it in simple terms, an active matrix display device comprises cells arranged in a crossed array of rows and columns, each cell comprising a pixel associated with an electronic switch. This switch is formed by an active element, which may be a transistor or a diode, that is electrically connected to a column of the array and connects the pixel to a row of the array. These cells are all identical. They are obtained from the same pattern at each level of manufacture. These cells form the active zone of the matrix circuit. Control and addressing circuits for the rows and columns of the matrix are positioned at the periphery. These circuits form the peripheral zone of the matrix.
Since the active zone of the active matrices for high-density screens has a large surface area, far greater than the size of the exposure fields, their manufacture requires the use of a stepper for the transfer and exposure, several times, of the same mask or reticle through which the patterns of the cells are formed. In a typical example, to make a display screen sized 600 by 600, the same mask is transferred nine times to cover the entire surface of the active zone.
2. Description of the Prior Art
FIG. 1a shows a glass plate 1 on which four display screens E1 to E4 are made. The active zone mask is transferred nine times for each screen, corresponding on the plate to the insulated fields marked ZA. In this example, the peripheral zone is obtained by means of four peripheral masks: the corresponding exposed fields are marked T-B for “Top-Bottom”, corresponding to the peripheral column control part; RL for “Right-Left”, corresponding to the peripheral row control part of the matrix and C-T (pour Corner-Top) and C-B (for Corner-Bottom) corresponding respectively to the upper and lower corners in the peripheral zone of the screens. The screens are then separated.
In practice, there is a stack of conductive layers and insulating layers to form an active matrix cell. For each of the levels of manufacture, there is a set of masks with designs corresponding to the level concerned. If we take the example of a thin-film transistor or TFT active matrix cell, shown in a top view in FIG. 1b and in longitudinal section views along A1–A2, A2–A3 and A3–A4 in FIGS. 1c to 1e, the conductive layers have different metal layers, for example a titanium-molybdenum (TiMo) layer for the selection rows and the gates of the transistors, a molybdenum (Moly) layer b for the sources and drains of the transistors and the columns of the matrix, a transparent layer c made of indium-tin oxide (ITO) for the pixel electrodes and a thin layer d made of semiconductor material (intrinsic amorphous silicon, n+ doped material, etc.) for the channel of the transistors. There are also different layers of insulating material f, g (nitride, etc) on which, in particular, peripheral contacts are etched. For each of these layers, there is a deposit of the layer of material, followed by a layer of photosensitive resin, then photo-exposure through a mask or reticle, development and etching. Each of these layers therefore has a corresponding masking level, with a corresponding set of masks. In the example shown in FIG. 1b, we thus have six masking levels, to which a seventh level is generally added. This seventh level corresponds to an (organic) insulating opaque level that is etched above the insulating layer g, to serve as an “LBL” (Light Blocking Layer) optical mask above each transistor (FIG. 1b). These masks are formed on one or more reticles, according to the place available on these reticles and the applicable design and layout rules. For the manufacturing level shown in FIG. 1a, the masks used are for example the set of masks [M-ZA, M-TB, M-RL, M-C] shown in FIG. 1f, and formed on three reticles, r1, r2 and r3. The corner mask M-C comprises the designs for the upper corner and lower corner corresponding to the exposed fields C-T and C-B in FIG. 1a. A particular feature of the mask of the active zone is that it is formed by a design of identical patterns, distributed in a crossed array of rows and columns, each pattern corresponding to a cell of the matrix.
In a step for the exposure of the designs or patterns of the active zone, the same mask M-ZA is thus transferred successively several times, nine times in the example shown, so as to cover the entire surface of the active zone. This transfer is made edge to edge, along the rows and the columns. It is done with a certain degree of imprecision corresponding to the tolerance of positioning of the stepper used and to the defects related to photo-exposure. These defects are especially edge effects, magnification, optical aberration, rotation and x-axis or y-axis misalignment of the mask. These defects are variable form one exposure field to another. They cause imprecision in the reproduction of the image or design at the boundary or junction between two successive exposure fields. This is the defect known as the “stepper pattern” defect which acts on the electro-optical qualities of the display device. This defect corresponds to the visual impact of a variation of the stray capacitive couplings of the cells of the matrix located at the boundary between two successive exposure fields. For example, in the case of a thin-field transistor (TFT) active matrix, there are unwanted couplings between the gate (row), the source (column) and the pixel electrode or the drain. These couplings are sensitive to the defects of the designs of the gates, the semiconductor layers, columns and drains of the TFTs and pixel electrodes. Along the boundary between two exposure fields, these couplings may have values that diverge from the mean value of the couplings in the very interior of the exposure fields.
In a screen without any “stepper pattern” flaw, the value of the couplings is uniform throughout the surface of the screen. The variation in the values of the couplings between two exposure zones due to the misalignment of the exposed designs on the matrix acts on the luminance of the concerned pixels along the boundaries between the different exposure zones. The real voltage applied to the terminals of the corresponding pixels will therefore be different along these boundaries, and this is something that will be seen visually: this is the stepper pattern.
If a voltage V is applied to the terminals E and CE of a pixel as shown schematically in FIG. 2b, in the case of a TN (twisted nematic) liquid crystal, the luminance is highly sensitive to the amplitude of this voltage. As shown in FIG. 2a which shows a luminance curve as a function of the amplitude of the voltage, L=f(V), it is in the zone of the gray levels that the variation is very great. Thus, with fields insulated according to the prior art, a luminance curve that is discontinuous is obtained on the active zone: the curve is not joined at the passage between the exposure boundaries. In practice, this break implies difficulty in accurately restoring the gray levels. On a display screen, the junctions between the different exposure fields of the active zone can be seen visually, as illustrated in FIG. 3a. This is because of the breaks in the luminance curve, for example of the type shown in FIG. 3b, along the x-axis. There is therefore a deterioration in visual quality.
There is a known method according to which, to reduce the effects of the mask-positioning errors, the boundaries between the exposure fields are shifted from one exposure level to another. In other words, the boundaries are not reproduced at the same positions from one level to the other. When this is done, the electro-optical effects due to the positioning errors are distributed by sliding the exposure boundaries on the surface of the matrix instead of putting them always the same place.
In the invention, another solution is sought to efficiently reduce the harmful effects due to the successive transfers of the active zone mask.
In the invention, a method is sought for the manufacture of an active matrix enabling a significant improvement in the visual quality of the display screens.
The idea on which the invention is based lies in transferring the active zone mask so that there is a non-zero overlap zone between two successive exposure fields. If a part of the active zone of the matrix is exposed a first time, the mask is positioned for the next exposure so as to overlap the first exposed field on a certain width, corresponding to the width of a peripheral zone defined on the mask. In this peripheral zone of the mask, the design to be made is incomplete or fuzzy.
A mask (or reticle) of an active matrix can be described as a a square-ruled layout or grid, in which each intersection of a row and the column is a dot that corresponds to an active matrix cell. This dot is actually an elementary pattern of the design (positive or negative) to be made on a cell for a level of etching considered. Thus, there is a design of identical patterns arranged in a crossed array of rows and columns corresponding to the crossed array of rows and columns of the matrix.
According to the invention, a central zone and a peripheral zone are defined on this mask. The elementary pattern is designed at each of the dots of the central zone of the mask. In the peripheral zone, this pattern is not drawn at certain dots. These dots are distributed randomly or pseudo-randomly.
The mask thus defined is transferred by causing the peripheral zone between two successive exposure fields to be overlapped. Each exposure thus provides a part of the patterns in the overlapped zones on the matrix. Preferably, each exposure provides the same density of patterns. Finally, the designs of the patterns in the overlap zones complete each other. In the overlap zones on the active matrix, there is a random spatial distribution of the patterns provided by the different exposures corresponding to the random distribution of the pattern-free dots between the different parts of the peripheral zone of the mask.
With this method of exposure, there is no longer any sharp boundary between two exposure fields, corresponding to an edge-to-edge transfer as in the prior art. On the contrary, there is a fuzzy boundary due to the width of the overlap zone, and the random or pseudo-random character of the spatial distribution of the patterns in this zone.
The method of overlapping according to the invention can be applied especially to the layers that play a direct role in the above-mentioned problems. In one example of a TFT active matrix, as illustrated in FIG. 1b, the method can be applied especially to the three masking levels of the layers b, d and c respectively corresponding to the columns, the channels of the thin-film transistors (TFTs) and the ITO pixel electrodes. More generally, this method can be applied to each of the masking levels of the active matrix, for example the seven levels described in detail with reference to FIG. 1b. 